rxgen: Don't overflow PackageIndex

PackageIndex++ returns the pre-index value of PackageIndex, so the
error statement isn't run when PackageIndex == MAX_PACKAGES. This
means we go on to overflow all of the arrays that are MAX_PACKAGES
in size.

Caught by coverity (#985583, #985584, #985585, #985586,
                    #985587, #985588, #985589)

Change-Id: If81f9ff89edc4cfd56677ff51cea71281ebe1e3b
Reviewed-on: http://gerrit.openafs.org/9325
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>